This invention relates to a method of providing a compressible slurry of contaminated, ion exchange bead resin, in the process of compression dewatering of such materials, prior to storage as solid waste.
Storage of nuclear contaminated wastes presents many problems. Naturally, if the waste can be compacted substantially, less burial space will be required. Such compaction can be easily handled for solids.
In the area of liquid type waste, radioactive slurries generated by nuclear reactor power plants, governmental operations, hospitals and the like must be disposed of at burial sites licensed by the U.S. government. Often these wastes are in the form of spent ion-exchange bead resins, spent ion exchange powdered resins, filter media, waste sludge, chemical precipitates, and similar granular-type slurry media which result from water treatment processes in the facilities generating the wastes. The use of ion exchange resins to capture a wide variety of metals, such as Pb, Rb, Cs, Ba, Sr, is taught in U.S. Pat. No. 5,242,503 (Grant et al.).
In nuclear reactor power plants, the liquid moderator circulating through the reactor typically contains dissolved and suspended radioactive solids. These solids usually are fission and corrosion products formed in the plant piping system and plant equipment through which the moderator circulates. The radioactive liquid moderator should be kept reasonably free from such fission and corrosion products during plant operation because the radiation fields surrounding the reactor have to be maintained at a low level for health reasons. Therefore, the liquid moderator is typically recirculated and filtered through ion exchange resin beds to remove the dissolved and suspended radioactive solids.
An ion exchange resin is usually a synthetic material or a natural or synthetic mineral that adsorbs an ion from solution in exchange for a less strongly held ion such as a sulfonic, carboxylic, phenolic, or substituted amino group that previously formed part of the structure of the resin. The ion exchange material can be in spherical "bead" form, usually 300 micrometers to 1000 micrometers diameter or finer "powdered" form, usually 10 micrometers to 400 micrometers diameter. After the accumulation of radioactive material in the resin bed reaches a predetermined level, the resin is typically removed for transport to an appropriate disposal site. However, such spent ion exchange resin bead slurry or spent ion exchange resin powder slurry contains a substantial amount of water and accumulated radionuclides. Some of these radionuclides may have a half-life of several hundred years and therefore should be isolated until the radiation emitting from the radionuclides has decreased to an acceptable level.
Burial of such low-level radioactive slurries is a relatively inexpensive means for isolating the radio-nuclides and for providing adequate long-term shielding. However, such burial raises the possibility of leaching by water and the possibility of contamination of nearby ground water. Therefore, when burial is used as means for waste disposal, it is desirable to eliminate water from the waste to reduce its volume and to compact the waste in waterproof and generally leak-tight containers before burial to reduce the risk of contamination of nearby ground water.
In the area of dry compaction, U.S. Pat. No. 4,452,733 (Horiuchi et al.), relating to contaminated slurries of ion exchange resins, teaches drying and milling the resins in a centrifugal membrane drier to provide dry contaminated resin and evaporated steam. Also, U.S. Pat. No. 4,762,647 (Smeltzer et al.), discloses treating low level radioactive contaminated slurry mixtures of ion exchange powder or ion exchange beads with 40 weight % to 70 weight % cellulose filter aid, by removing all the water from the mixture and then compacting at 141 kg/cm.sup.2 (2,000 psi) and 230.degree. C., to form a sintered monolith.
An apparatus for removal of liquid from a waste burial container holding a slurry of waste material, without first drying, is taught by U.S. Pat. No. 4,582,099 (McDaniel et al.). In U.S. Pat. No. 4,836,937 (Homer), contaminated, spent, ion-exchange resin powder slurries were dewatered without any initial drying. Here, the powder slurry was directly passed into a vessel containing filters and a bag of pliant material which, when the vessel was evacuated, could compress the ion exchange powder against the filters. In this regard, U.S. Pat. No. 4,952,339 (Temus et al.) taught that 40 to 50% volume reduction, involving complete water removal from contaminated ion exchange resins, could cause burial containers to burst due to ion exchange resin swelling caused by ground water leakage into the container. There ion exchange resins were not dried beyond their burial equilibrium endpoint so that only enough water was removed to equal saturated conditions experienced by the resin at its burial temperature. These dewatering methods were improved upon in U.S. Pat. Nos. 5,022,995 and 5,143,615 (both Roy et al.) which teach 20 to 30 volume % reduction dewatering of contaminated ion exchange powder material, filter media, waste sludge, and other granular type slurries by compression as well as suction.
What is needed is a method to economically treat, in a continuous fashion and without the expensive steps of first drying and handling contaminated vapor, liquid slurries of the larger, contaminated, depleted, ion exchange resin beads, which appear incompressible in large measure. Thus, the invention of this application relates primarily to reducing the volume of resin beads rather than already size reduced resin powder. It is one of the main objects of the invention to provide such a method and apparatus.